1. Field of the Invention
This invention relates to rubber compositions for tires, and more particularly to a super-hard rubber composition usable as a bead filler rubber as well as a passenger car tire using the same.
2. Related Art Statement
As to the bead portion structure of the radial tire, there have made various investigations for satisfying rigidity and durability required as a tire.
It has been attempted to improve the running performances and durability of the tire by arranging the bead reinforcing layer in the bead portion.
On the other hand, it has been well-known that the running performances are improved by arranging a super-hard rubber in the bead portion as disclosed in Japanese Utility Model Application Publication No. 47-16084, French Patent No. 1,260,138, U.S. Pat. No. 4,067,373 and the like.
However, satisfactory solution on the above problems have not yet been obtained by these conventional techniques. For this end, the inventors have previously proposed a rubber composition for bead filler obtained by compounding novolak type phenolic resin and/or novolak type modified phenolic resin modified with an oil or the like and hexamethylene tetramine (hereinafter referred to as hexamine simply) together with carbon black into natural rubber, polybutadiene rubber or the like as disclosed in Japanese Patent Application Publication No. 57-30856.
Moreover, the above Japanese Patent Application Publication No. 57-30856 discloses that a resin modified with an oil such as rosin oil, tall oil, cashew nut oil, linol oil, olein oil or the like, an aromatic hydrocarbon such as xylene or the like, or a rubber such as nitril rubber or the like as a novolak type modified phenolic resin is compounded into polyisoprene rubber or the like.
In the arrangement of the bead reinforcing layer, the production steps of tire becomes large, and the productivity is remarkably poor.
In the arrangement of the super-hard rubber, it is hardly considered that the function as the bead filler rubber is sufficiently developed under complicated input during the running of the tire to provide the durability required as a tire.
In order to solve these problems, in the technique proposed by the inventors, the novolak resin is cured in rubber through a curing agent such as hexamine, hexamethoxymethyl melamine or the like. Owing to the curing in rubber, the curing efficiency is poor as compared with the resin molding usually conducted only by mixing the resin and the curing agent, and a large amount of resin remains at an unreacted state. For this end, it is necessary to increase the compounding amount of the resin or to increase the amount of the curing agent for obtaining a desired hardness as a bead filler rubber.
However, when the compounding amount of the resin is increased, the amount of unreacted resin becomes naturally large. Such a unreacted resin degrades mechanical properties, particularly fatigue life and creep property of the rubber composition as a mere foreign matter and increases the heat build-up to shorten the fracture life of the tire.
On the other hand, when hexamine is used as a curing agent, if the amount of the curing agent is increased, the decrease of tenacity is caused due to the amine degradation of polyester fiber widely used as a reinforcing cord for adjoining carcass ply during the vulcanization of the tire or during the running of the tire. Particularly, this comes into problem when the tire is vulcanized at high temperature. When hexamethoxymethyl melamine is used as a curing agent, the decrease of tenacity in the polyester fiber is not caused, but there is still the problem due to the presence of the unreacted resin as previously mentioned because the curing efficiency is lower than that of hexamine.
Thus, the compounding of novolak type phenolic resin and curing agent such as hexamine or the like into rubber is a general means for increasing the hardness of rubber, but according to this means, a significant amount of unreacted resin remains in rubber because the curing reaction efficiency of the resin is poor, so that the mechanical properties of rubber, particularly dynamic modulus E', fatigue life, creep property, heat build-up and the like are degraded to give unsatisfactory rubber properties. Therefore, when such a conventional rubber composition is used as a bead filler rubber, the tire performances are still insufficient.